Telomeres are specialized nucleoprotein complexes that constitute the ends of eukaryotic chromosomes and protect chromosomes from degradation and end-to-end fusion. When telomeres are absent, the instability of non- telomeric chromosomal ends leads to -chromosome loss. In addition, telomeres are required for the complete replication of chromosomes. The complete replication of telomeres is dependent upon the ribonucleoprotein telomerase. Telomerase is a reverse-transcriptase, with an integral RNA that serves as a template to synthesize telomere DNA sequences. While telomerase activity has been detected in a number of species, including humans, the only component that has been isolated so far is the template RNA. Up until now, the RNA has only been found in ciliated protozoans. We have recently identified the gene, TLC1, which encodes the template RNA component of telomerase in Saccharomyces cerevisiae. We propose to take advantage of this discovery and use a combination of genetic and biochemical approaches to identify the other components of telomerase. In addition we will use mutations in the TLC1 gene to analyze the function and regulation of telomerase in vivo. Our long term goal is to develop a mechanistic understanding of telomerase activity and its regulation. These studies should reveal important aspects about telomerase regulation and telomere replication in yeast. Given the conservation between yeast and humans, these studies will be applicable to humans. Thus these studies are relevant to human disease, since a loss of telomerase activity appears to play an important role in the aging of somatic cells and a gain of telomerase activity may aid in achieving immortalization of tumor cells.